One step closer to the mysterious dark matter

Maja Llena Garde is a PhD student in the Cosmology, Astroparticle Physics and String Theory group at the Oskar Klein Centre. She is involved in the Fermi-LAT collaboration together with her supervisor Jan Conrad. The Large Area Telescope (LAT) is a space based imaging high-energy gamma-ray telescope launched in orbit in June 2008.
Their recent paper on Dark Matter has attracted some attention, thus we asked Maja to tell us more about it.

Fermi satellite
photo by NASA och General Dynamics
It has been shown that about 25% of our universe consists of dark matter, i.e. an invisible type of matter that neither emit nor reflect electromagnetic radiation. Physicists around the world try to figure out what this mysterious matter is.

One way to search for dark matter is by looking at gamma rays. If the dark matter consists of weakly interacting massive particles (WIMPs), then they can self-annihilate or decay into standard model particles, and this process will give rise to a gamma-ray signal. There are many places to search for this gamma-ray signal, and one example is dwarf spheroidal galaxies. These are satellite galaxies to the Milky Way and are known to have a large dark matter content. They are free from other gamma-ray sources, quite near-by and many of them are situated far away from the galactic plane so the galactic foreground is low. These properties make them excellent targets for dark matter searches. But the gamma-ray signal is expected to be very low.

The Fermi-LAT collaboration is using a new method to search for dark matter. Instead of looking at one target at the time, we make a joint likelihood analysis looking at 10 dwarf spheroidal galaxies at the same time. By using this joint likelihood approach we get higher statistics and we are less sensitive to background fluctuations, so we can set more robust limits on the dark matter parameter space. Another novelty in our analysis is that we adequately take into account the uncertainty in the dark matter density in these galaxies. Unfortunately, no signal has been found, but this powerful statistical method allows us to strongly disfavour dark matter models that were inaccessible before, and our constraints are among the strongest obtained to date.

This work will be publisched in Physical Review Letters, and can also be found on the arxiv: 1108.3546)

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